Mistress 9 is pure evil. She is frightening, cruel, and hateful; she took over Hotaru's body and deprived Chibiusa of her heart without a second thought. Whenever Hotaru's spirit fights her, she rants in anger and lashes back madly. Like any good villain, she gloats over the defeat of her enemies. And yet she has no depth.
Mistress 9, you see, exists for only one purpose - to gain enough power to open the doorway for Pharoah 90. To that end, she devotes all her energy and thoughts. She has no other wants, no other desires; once she opens the doorway for the Pharoah, she can subsume herself in his existence, and rule the world with him.
She took over Hotaru's body, and so they share some resemblance - but that is all. When Mistress 9 inhabited Hotaru's body, Hotaru's hair grew incredibly long; black and straight, it falls to the floor and pools around her feet. Mistress 9's evil also twists Hotaru's sweet features into a horrible parody of what they used to be. She wears a long skirt with a tight bodice, and beads lie around her shoulders and dangle from her ears.
She took over Hotaru's body just as Chibiusa arrived; when Chibiusa got within arm's reach, Mistress 9 reached out and tore the Silver Crystal from Chibiusa's chest. Chibiusa fell back, dying; Mistress 9 cackled gleefully and drew on the power of the Silver Crystal. However, Hotaru fought back inside her; Mistress 9 was unable to use the full power of the Silver Crystal, nor was she able to free herself of Hotaru's influence.
Finally, Hotaru's spirit left her former body and restored Chibiusa's heart. When Mistress 9 had complete control of the body, she mutated into a huge black monster and opened the doorway for Pharoah 90, despite the outer senshi's efforts to hold her back.
Mistress 9 was incredibly powerful; none of the senshi were able to hurt her. She merely absorbed their attacks and grew even stronger. However, just as things were going well, she felt an explosive pain in her forehead, and a form inside a ball of light burst out of her head. It was the senshi of destruction, Sailorsaturn. When Saturn hit Mistress 9 with her glaive, Mistress 9 screamed as she felt her energy draining away. Sailorsaturn forced Mistress 9 into the air to join her Master, both of them impotent and trapped in the sky. Sailorsaturn forced them through the doorway back to their own world, and went in after them.
Sailorpluto raised the Garnet Orb and lifted her voice in command. "Dark Dome Close!" she cried. The doorway closed, sealing Mistress 9 and Pharoah 90 away forever.